What is "Orosuke"? Is it Sasuke with some of Orochimaru's powers, exactly what we have seen fighting Itachi? Or is it Orochimaru with Sasuke's Sharingan and body? The latter beats Itachi with the help of "Orobuto" – the former defeats Itachi on his own.
The second type of "Orosuke" uses yamata aggressively, probably is restored to full grace and can use the jutsu he used to force just-off-prime Sarutobi to the ground in an instant (often forgotten panel, during Orochimaru's escape – Sarutobi was a little unwilling to stop him, but the jutsu Orochimaru used looked impressive), refuses to die, refuses to be subject to Itachi's genjutsu, throws around fast Kusanagi attacks from underground… And will not lose to amaterasu and susano'o.
This is pretty much an obvious one, yeah. When Sasuke with a few of Orochimaru's powers came so close, full and prime Orochimaru with Sasuke's Sharingan is going to do just as well or better, and "Orobuto" makes this a certainty against an opponent who really doesn't want to fight two-on-one.